Ok I checked up a bit on it, and as far as I can tell (although I'm by no means an expert) it seems like those blocks are seen as a single block with its own id by the minecraft engine. Then they have item data (which many blocks have) which determines how it actually looks, but that should only be an issue for the client I guess.
So as far as the server goes it shouldn't be a major problem, but as with anything, if used in excess it could probably slow down weaker clients due to the more advanced geometry. But then again, for example a pipe with a pipe cover is probably as bad or often even worse in that regard.
EVE Online characters:
Daedalus II ex-CEO TOGOA, Daedalus III slave of Daedalus II
Little Molok Asura Engineer
"Each day I don't get an attempt on my life by a robot from the future I see as a personal failure"
Last edited by DaedalusAlpha; 22nd January 2013 at 04:25 AM.